The advantages of utilizing magnesium oxide as an electrical insulator are well known and because of the many advantages which derive from using this compound, a wide variety of electrical insulators are presently formed from materials which contain a substantial amount of magnesium oxide. One typical prior art procedure for producing such insulators is to introduce a raw material containing magnesium oxide into a three phase electric arc furnace. The raw materials generally used are magnesium ores such as magnesite, periclase, dolomite, kieserite, epsomite, carnalite, asbestos, talc and other minerals. These ores generally are calcined prior to fusion so that in substance the oxides of the ores are fused. The three phase furnace contains an eletric arc located in the center of the furnace to fuse the magnesium oxide.
However, with this apparatus the particulate refractory magnesium oxide ores when heated by the arc form a molten mass which becomes the inner core which is surrounded by a layer of material encapsulating the mass which is generally referred to as the "crust" and "fall-off." Only the portion of the foregoing mass referred to as the core is immediately usable in subsequent stages of the insulator producing process since it is the only part having been completely fused and only in the core have a sufficient amount of impurities been driven off. The crust and fall-off must be removed from the core, and some of it may be recycled back into the electric arc furnace.
The reason why such a mass is formed is that electrically conductive materials tend to form part of the arc circuit, with the result that the material is heated first on the inside and then, as the heat is conducted outwardly of each particle, the outside is heated last. Another result of this inside-outwardly heat transfer is the uneven heating of the particles and, therefore, the material as a whole tends to form a crust of lower quality which acts as a heat insulator to protect the wall from overheating. This crust also tends to trap the impurities in the particles which are vaporized near the center and redeposited (by sublimation) in the cooler crust. This disadvantage also exists in electrically nonconductive materials, such as unfused magnesium oxide, because such materials tend to interfere with the conductivity and enclose the electric arc so as to reduce the heating effect by a reduction in the flow of electric current even to the point of extinguishing the arc on occasions.
In the foregoing prior art process for producing usable insulating particles from magnesium oxide, the core material once cooled is broken with suitable equipment, such as a hammer, or a jaw crusher. The crushed material thereafter is passed into an impactor to produce a substantially pure magnesium oxide material or "grain" as it is commonly called. The grain is then passed through screens to remove oversized particles and is also subjected to a magnetic field which removes any magnetic impurities.
After a series of additional steps, which need not be described in order to understand the present invention, the magnesium oxide is classified into various sizes, blended and packaged for further use.
Since the range of the size of grain produced is advantageously narrower by utilizing the present process as compared to the known prior art processes, a description of the significance of grain size follows.
The ideal magnesium oxide insulator would be composed of 100% pure magnesium oxide particles which may easily and readily be closely packed to produce a very dense pure substance. However, the achievement of such a pure insulator would be an extremely costly process. Therefore, some compromise has to be made as to whether or not to obtain an expensive pure insulating material or as an alternative an acceptable insulating material at a more reasonable cost.
The physical property normally utilized to evaluate the usefulness of a magnesium oxide containing material is density. Although the density of pure magnesium oxide is approximately 3.58, grains of magnesium oxide having a bulk density within the range of between 2.38-2.42 g/cc are usable for forming insulators. A typical range obtainable from the foregoing prior art process is 2.38-2.40 g/cc with 2.38 g/cc being the most common bulk density of the packed grains produced from the foregoing process.
In order to obtain a usable insulating material from the foregoing process, within the foregoing bulk desity range of 2.38-2.42 g/cc, the magnesium oxide grains are classified into various sizes according to minimum cross-sectional dimensions.
The prior art process results in grains with sizes from 40 mesh to less than 325 mesh, (U.S. Sieve Series), making it necessary to blend the grains by particle size, such as in accordance with the following table illustrating a typical blend:
Table 1 ______________________________________ % of Grain Utilized to Mesh Size Produce Usable Blend ______________________________________ 40 20 50 15 60 80 52 200 325 8 &lt;325 5 100 ______________________________________
It is, of course, desirable to provide a process wherein the size of the grain produced is within a narrower range and where substantially all of the grains produced in the melting step can be used to form insulators. It is also desirable to reduce the number of particle size-reduction steps in the foregoing process, notably the hammering, crushing and impacting steps and yield a material from which insulators could be formed.
A further disadvantage which results from utilizing electric arc type furnaces is that the arc is developed between the electrodes and thus does not extend beyond this very limited area. A still further disadvantage of electric arc melting is that incomplete combustion of the carbon electrodes permits introduction of carbon as an impurity to the solidified fused magnesia, which impurity would have to be oxidized out in a post-fusion step to obtain the highest quality insulation.
It has been suggested that a flame type apparatus would overcome the disadvantage of the electric arc furnace created by the limited heating area of the arc. The word "flame" has been used in this art to denote the heating zone produced by one or more gases, as contrasted with the word "furnace" used to refer to an electric arc device. However, a flame type apparatus has a temperature limitation which renders it impractical. The hottest flame type apparatus has been the oxygen-acetylene combination, and the temperature achieved by it is limited to about 5,000.degree. F. Somewhat higher temperatures may be achieved by other gas combinations, or by preheating the fuels, but those generally are not available commercially and are uneconomical.
Many other devices have been proposed for overcoming the limited heating area disadvantage of the electric arc. However, such devices also have not been particularly effective or commercially feasible.
Therefore, the electric arc apparatus has been and still is the principal source of the heat required for producing fused magnesium oxide for electrical insulators.
In any fusion process for producing high quality fused magnesium oxide, regardless of the type of furnace used, the system must be capable of fusing the material and maintaining the particulate nature of the material. Furthermore, fusing must be accomplished without contamination from carbon and must result in the production of a grain wherein each grain has substantially the same chemical composition and consistency of electrical resistance. It should be further possible to fuse a material and obtain a flow characteristic of a liquid.